1. Field of the Invention
The present invention relates generally to the field of scintillation counting and more particularly to a method and apparatus for efficient storage of pulse height data acquired by a scintillation counter.
2. Description of Related Art
Scintillation counting techniques are well known for measuring the radioactivity of samples containing radionuclides which permits identification of the samples. For example, in liquid scintillation, a radioactive sample, an alpha, beta or gamma emitter, is dissolved or suspended in a liquid scintillation medium. The liquid scintillation medium comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the liquid scintillation medium. The radioactive sample begins to disintegrate within the liquid scintillation medium. It is theorized that most of dynamic energy from the nuclear disintegrations of the radioactive sample is absorbed by the solvent and then transferred to the solute which emits photons as visible light flashes or scintillations. The amount of photons emitted from a scintillation is proportional to the energy of the corresponding nuclear disintegration and is characteristic of the sample.
A scintillation counter measures the relative intensities of scintillations occurring within a scintillation mixture. Typically, scintillations occurring within the scintillation mixture are detected by suitable photodetector which produces output pulses having pulse heights proportional to the number of photons generated by the corresponding scintillations. The scintillation counter counts the pulse in a plurality of pulse height channels or windows having upper and lower pulse height limits that together span a predetermined range of pulse heights. The counts accumulated in the windows may be plotted with respect to corresponding pulse heights to provide a pulse height spectrum representing the energy spectrum of the nuclear radiation emitted by the radioactive sample.
Prior to the development of low cost multichannel analyzers such as the modern analog-to-digital integrated circuit, the analysis of scintillation pulse height data was done by using several discrete counting windows or channels, with either linear or logarithmic amplification. See Horrocks, D. L. "Applications of Liquid Scintillation Counting" (1974), Chapter IV, Academic Press. An advantage of using logarithmic amplification was that only one amplifier was required to process full range of the pulse height data. In addition, discussions of pulse height spectra were often facilitated when they were plotted versus the logarithm of energy full range, since tritium H3 has a maximum energy of only 18 KeV compared to P32, which has a maximum energy of 1.7 MeV.
Since the introduction of now-familiar analog-to-digital converters, multichannel analysis of pulse height data is possible, instead of being limited to a few discrete counting windows.
It is a well known concern in the scintillation counting art that materials present in the scintillation mixture can decrease the number of photons reaching the photodetector for a given nuclear disintegration. For example, the emission of photons in a liquid scintillation solution can be prevented or emitted photons can be absorbed. Furthermore, some scintillation events can be reduced to a level which is below the minimum detection level of the photomultiplier. Such effects are commonly referred to as "quenching" and in each case result in the reduction in the number of photons detectable by the photodetector. Because quenching decreases the number of photons applied to the photodetector, the result is that the number of counts per unit time detected by the photodetector for a quenched sample is decreased as compared with an otherwise identical unquenched sample. The result of quench, therefore, is to shift the pulse height spectrum of the quenched scintillation sample along the pulse height axis to lower pulse height values, and this is commonly referred to as "pulse height shift".
In order to correct for the effect of sample quench, systems have been developed for determining the degree of quench in a sample and for adjusting the relative position of the pulse height spectrum and the window in which samples scintillations are measured by an amount corresponding to the degree of sample quench. Such automatic quench compensation methods, in effect, operate to re-establish the correct relative position of the pulse height spectrum in the measuring window. Measurement of the degree of sample quench for use in quench compensation methods can be performed by any of numerous known techniques. See Horrocks, supra at Chapter X. A highly desirable quench determination method, termed the "H-number technique", is disclosed in U.S. Pat. No. 4,075,480 to Horrocks which is assigned to the assignee of the present invention. In the H-number technique, a liquid scintillation sample is irradiated by a standard source (e.g. cesium 137) to produce a Compton scattered pulse height spectrum. The relative shift of a unique point (typically a point of inflection) on the leading edge (or Compton edge) of the Compton spectrum between the irradiated quenched sample and a similarly irradiated standard sample provides a measure of the degree of quench. Implementation of the foregoing quench correction method requires a pulse height spectrum of the standard sample be stored in the memory of a computer, at least temporarily, for later comparison with the spectrum of the quenched sample. In addition, a large number of channels must be used to obtain sufficient resolution to permit accurate identification of the point of inflection on the Compton edge. As a consequence, a large number of computer memory locations are required to store the results of the large number of measurements. Furthermore, the fact that the width of the Compton edge changes drastically with quench level means that a time consuming algorithm must be used for processing a large volume of data. In the past, logarithmic amplification results in bunching of data points at high energies, while linear amplification results in bunching of data points at low energies. The resolutions obtained in the determination of the point of inflection on the Compton edge often do not meet expectations.